The amplification of ultrashort laser pulses involves pulse width manipulation by chirped-pulse amplification (CPA). See U.S. Pat. No. 5,235,606 which is incorporated herein by reference in its entirety. A train of ultrashort pulses, on the order of less than one picosecond and of low pulse energy, are generated from an ultrafast laser oscillator. One pulse from the pulse train is selected as a seed pulse to be amplified to gain pulse energy in a laser amplifier. When amplifying the ultrashort pulse in a solid-state gain media, the pulse is stretched (chirped) in time before amplification. This is to avoid damage to the gain medium and other optical components in the amplifier that would result due to high laser intensity if an unstretched pulse were amplified. After amplification, the stretched pulse is recompressed, often back to its initial ultrashort pulse state by a pulse compressor. Therefore, an amplified ultrashort-pulse laser system based on chirped-pulse amplification consists of 4 basic elements: oscillator, stretcher, amplifier, and compressor. This is fully described in U.S. Pat. No. 5,235,606 and illustrated in FIG. 1 attached hereto.
The pulse stretcher and amplifier are key elements of a CPA laser system, and are what set it apart from other amplified laser systems. The most common pulse stretcher and amplifier use diffraction gratings for pulse width manipulation. The stretcher uses the dispersive properties of the diffraction grating to disperse the many different frequency components that make up the ultrashort pulse into different optical paths. Each path has different optical path length. At the output of the stretcher, all the frequency components are recombined to have the same optical path again. Because the different frequency components have different optical path in the stretcher, they arrive at the output at different times, since the speed of light in air is constant for these frequency components. The frequency components that have longer optical paths will arrive at the output later than those that have shorter optical paths, therefore the output pulse is stretched in time. Normally in a stretcher, the lower frequency components (longer wavelengths) have shorter optical paths than higher frequency components (shorter wavelengths), so they make up the leading edge of the stretched pulse, while higher frequency components form the trailing edge. In the compressor, the optical paths are reversed from the stretcher: lower frequency components take the longer optical paths, while higher frequency components have the shorter paths, canceling the pulse stretching effect of the stretcher. The output from the compressor is typically a pulse with the original ultrashort pulse width, although a different duration is also possible.
There are many implementations of pulse stretcher and compressor. FIG. 2 shows a conventional stretcher having a pair of gratings. FIG. 3 shows a conventional compressor having a pair of gratings. Conventional applications requiring a source of amplified ultrashort pulses are mainly reliant on Ti:sapphire-based lasers. Typically these regenerative amplifier systems produce sub-100-fs pulses with energies in the millijoule range and repetition rates of 1 kHz. However, because of their large size and costly pump sources, these Ti:sapphire amplifiers are unattractive for many commercial, medical and industrial uses.
Recent progress in high-power diode-laser technology and the advent of new laser materials such as Cr:LiSAF and Cr:LiSGaF have accelerated the development of cost-effective all-solid-state femtosecond oscillators, which are now beginning to replace traditional gas-laser-pumped femtosecond lasers such as dye or Ti:sapphire oscillators. However, little success has been achieved in replacing traditional femtosecond regenerative amplifiers with the new directly diode-pumped laser materials. These materials are limited by the low power of the currently available 670-nm diodes, their energy-storage capabilities (with a relatively low upper-state lifetime of .about.67 .mu.s), and severe thermal-lifetime quenching in Cr:LiSAF3.
There is a continuing need for improvements in apparatus and methods for generating high-power, ultrashort laser pulses. There is also a need to simplify the laser systems so that when a change in the laser wavelength is required, precise readjustment is less burdensome. It is also desirable to eliminate the problem of strictly matching grating pairs required for conventional stretcher and compressor, as illustrated in FIGS. 2 and 3 herein.